Porous sound absorbing structure

ABSTRACT

A porous sound absorbing structure  10  includes an exterior plate  1  and a closing plate  2  in order to suppress a decrease in sound absorbing performance in a wide frequency band. An interior plate  3  is arranged between the exterior plate  1  and the closing plate  2 , and air layers  4  and  5  are located between the exterior plate  1  and the interior plate  3  and between the interior plate  3  and the closing plate  2  respectively. The exterior plate  1 , the interior plate  3 , and the closing plate  2  are arranged while opposed to each other. Many through holes  1   a  and  3   a  are made in the exterior plate  1  and the closing plate  2  respectively. In the interior plate  3 , a plate thickness t 2  and a diameter φ 2  and an aperture ratio β 2  are set so as to generate an viscous effect in air passing through the through hole  3   a . The aperture ratio β 1  of the through hole  1   a  in the exterior plate  1  is set more than 3% and not more than 50%.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 11/578,666 filed Oct. 17, 2006, which is a national stageapplication of PCT/JP05/7322 filed Apr. 15, 2005. Priority is claimedbased on U.S. application Ser. No. 11/578,666 filed Oct. 17, 2006, whichclaims priority to PCT/JP05/7322 filed Apr. 15, 2005, which claimspriority to Japanese Patent Application No. 2004-135145 filed on Apr.30, 2004, all of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a porous sound absorbing structurewhich reduces sound from a noise emitting source.

BACKGROUND ART

Recently, a porous soundproof structure is becoming operational invarious fields. In the porous soundproof structure, a plate member inwhich a hole is not made and a plate member in which many through holesare made over a surface are arranged on an outside and an inside withrespect to a sound source while opposed to each other through an airlayer, and thereby a Helmholtz resonance principle is utilized toperform noise control. When the Helmholtz resonance principle isutilized, for example, a relationship of a general formulaf=(c/2π)×f{β/(t+1.6φ)d} holds, where f is a resonance frequency, c issound velocity, β is an aperture ratio, t is a plate thickness of theinside plate, φ is a diameter of the through hole, and d is an air layerthickness. Air in the portion of the through hole is drasticallyvibrated to the sound having a resonance frequency, and the sound havingthe resonance frequency is absorbed by friction. Therefore, the noisehaving a particular resonance frequency can efficiently be decreased.

However, when the Helmholtz resonance principle is used, there is aproblem that a soundproof effect is exerted only to the sound having theparticular resonance frequency while the soundproof effect is extremelylowered to the sounds having frequencies except for the resonancefrequency. In order to solve the problem, there is known a poroussoundproof structure in which a sound absorbing band is widened byproviding plural interior plates through the air layers. The interiorplate becomes the sound absorbing plate, and many through holes (microholes) are made in the interior plate. This enables the noises havingthe frequencies including the particular resonance frequency to beabsorbed, because the sound absorbing band is widened.

However, in the porous soundproof structure, the interior plate isexposed on a sound source side, and the diameter of the through hole isextremely small, so that sometimes clogging is generated in the throughhole. Furthermore, sometimes the diameter of the through hole cannot bedecreased due to constraints from other functions (drainage, painting,and the like). Thus, because the size of the through hole diameter issubject to the restraint, there is a problem that the frequency band inwhich the sound absorbing performance can be exerted becomes narrowed.

An object of the present invention is to provide a porous soundabsorbing structure which can sufficiently exert the sound absorbingperformance in the wide frequency band.

DISCLOSURE OF THE INVENTION

A porous sound absorbing structure according to one aspect of thepresent invention is a porous sound absorbing structure in which a firstoutside member and at least one inside member are arranged while opposedto each other, many through holes being made along a flat plane in theinside member, and a plate thickness of the inside member and a diameterand an aperture ratio of the through hole are set so as to generate anviscous effect in air passing through the through hole. In the poroussound absorbing structure, a second outside member is arranged whileopposed to the inside member, the second outside member being located onthe side opposite to the first outside member with respect to the insidemember, many through holes being made along the flat plane in the secondoutside member, and the aperture ratio of the through hole in the secondoutside member is more than 3% and not more than 50% (claim 1).

According to the porous sound absorbing structure according to thepresent invention, the aperture ratio of the second outside memberexceeds 3%, which promotes the conversion of the air vibration into theheat energy by the viscous effect of the through hole in the insidemember, and the sufficient sound absorbing performance can be exerted inthe wide frequency band. Additionally, because the aperture ratio of thesecond outside member becomes not more than 50%, the rigidity of thesecond outside member can be secured to some extent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse sectional view showing a porous sound absorbingstructure according to a first embodiment of the present invention;

FIG. 2 is a graph showing a relationship between an acoustic absorptioncoefficient and an aperture ratio of an exterior plate in the poroussound absorbing structure according to the first embodiment of thepresent invention;

FIG. 3 is a graph showing sound absorbing characteristics in the poroussound absorbing structure according to the first embodiment of thepresent invention;

FIG. 4 is a graph showing the sound absorbing characteristics in theporous sound absorbing structure according to the first embodiment ofthe present invention;

FIG. 5 is a graph showing the sound absorbing characteristics in theporous sound absorbing structure according to the first embodiment ofthe present invention;

FIG. 6 is a graph showing the sound absorbing characteristics in theporous sound absorbing structure according to the first embodiment ofthe present invention;

FIG. 7 is a transverse sectional view showing a porous sound absorbingstructure according to a second embodiment of the present invention;

FIG. 8 is a transverse sectional view showing a porous sound absorbingstructure according to a third embodiment of the present invention;

FIG. 9 is a transverse sectional view showing a porous sound absorbingstructure according to a fourth embodiment of the present invention;

FIG. 10 is a transverse sectional view showing a porous sound absorbingstructure according to a fifth embodiment of the present invention;

FIG. 11 is an explanatory view showing a state in which irregularity isformed in an exterior plate 1;

FIG. 12 is an explanatory view showing a state in which the irregularityis formed in an interior plate 3;

FIG. 13 is an explanatory view showing a state in which the irregularityis formed in the exterior plate 1 and the interior plate 3;

FIG. 14 is an explanatory view showing a state in which a corrugatedirregularity is formed in the interior plate 3;

FIG. 15 is an explanatory view showing a state in which an outside riband an inside rib are provided in the exterior plate 1 and the interiorplate 3 and the irregularities are formed in the exterior plate 1 andthe interior plate 3;

FIG. 16 is an explanatory view showing a state in which the corrugatedirregularity is formed in the interior plate 3 and the emboss isprovided in the exterior plate while the irregularity is formed in theexterior plate; and

FIG. 17 is an explanatory view showing a state in which theirregularities are formed in the exterior plate and the interior platewhen the interior plate is inclined.

BEST MODE FOR CARRYING OUT THE INVENTION

A porous sound absorbing structure according to another aspect of thepresent invention is a porous sound absorbing structure in which a firstoutside member and at least one inside member are arranged while opposedto each other, many through holes being made along a flat plane in theinside member, and a plate thickness of the inside member and a diameterand an aperture ratio of the through hole are set so as to generate anviscous effect in air passing through the through hole. In the poroussound absorbing structure, a second outside member is arranged whileopposed to the inside member, the second outside member being located onthe side opposite to the first outside member with respect to the insidemember, many through holes being made along the flat plane in the secondoutside member, and the diameter of the through hole in the secondoutside member exceeds 3 mm (claim 2).

A porous sound absorbing structure according to another aspect of thepresent invention is a porous sound absorbing structure which includes afirst outside member; a second outside member which is arranged whileopposed to the first outside member, many through holes being made alonga flat plane in the second outside member; and at least one insidemember which is arranged while opposed to the first outside member andthe second outside member such that the inside member is located betweenthe first outside member and the second outside member, many throughholes being made along the flat plane in the second outside member. Inthe porous sound absorbing structure, a plate thickness of the insidemember and a diameter and an aperture ratio of the through hole are setso as to generate an viscous effect in air passing through the throughhole, and the aperture ratio of the through hole in the second outsidemember is more than 3% and not more than 50% (claim 4).

A porous sound absorbing structure according to another aspect of thepresent invention is a porous sound absorbing structure which includes afirst outside member; a second outside member which is arranged whileopposed to the first outside member, many through holes being made alonga flat plane in the second outside member; and at least one insidemember which is arranged while opposed to the first outside member andthe second outside member such that the inside member is located betweenthe first outside member and the second outside member, many throughholes being made along the flat plane in the second outside member. Inthe porous sound absorbing structure, a plate thickness of the insidemember and a diameter and an aperture ratio of the through hole are setso as to generate an viscous effect in air passing through the throughhole, and the diameter of the through hole in the second outside memberexceeds 3 mm (claim 5).

According to the porous sound absorbing structure of the presentinvention, the aperture ratio of the second outside member exceeds 3%,which promotes the conversion of the air vibration into the heat energyby the viscous effect of the through hole in the inside member, and thesufficient sound absorbing performance can be exerted in the widefrequency band. Because the aperture ratio of the second outside memberbecomes not more than 50%, the rigidity of the second outside member canbe secured to some extent. Because the diameter of the through hole inthe second outside member exceeds 3 mm, the diameter of the secondoutside member can relatively be increased such that the clogging is notgenerated in the through hole and such that the constraints due to otherfunctions (drainage and painting) can sufficiently be satisfied.Therefore, the sound absorbing performance can be maintained in theinside member arranged inside the second outside member.

A porous sound absorbing structure according to another aspect of thepresent invention is a porous sound absorbing structure in which a firstoutside member and at least one inside member are arranged while opposedto each other, many through holes being made along a flat plane in theinside member, and a plate thickness of the inside member and a diameterand an aperture ratio of the through hole are set so as to generate anviscous effect in air passing through the through hole. In the poroussound absorbing structure, a second outside member is arranged whileopposed to the inside member, the second outside member being located onthe side opposite to the first outside member with respect to the insidemember, many through holes being made along the flat plane in the secondoutside member, the aperture ratio of the through hole in the secondoutside member is more than 3% and not more than 50%, and the diameterof the through hole in the second outside member exceeds 3 mm (claim 3).

A porous sound absorbing structure according to another aspect of thepresent invention is a porous sound absorbing structure which includes afirst outside member; a second outside member which is arranged whileopposed to the first outside member, many through holes being made alonga flat plane in the second outside member; and at least one insidemember which is arranged while opposed to the first outside member andthe second outside member such that the inside member is located betweenthe first outside member and the second outside member, many throughholes being made along the flat plane in the second outside member. Inthe porous sound absorbing structure, a plate thickness of the insidemember and a diameter and an aperture ratio of the through hole are setso as to generate an viscous effect in air passing through the throughhole, the aperture ratio of the through hole in the second outsidemember is more than 3% and not more than 50%, and the diameter of thethrough hole in the second outside member exceeds 3 mm (claim 6).

According to the porous sound absorbing structure of the presentinvention, because the diameter of the through hole in the secondoutside member exceeds 3 mm, which promotes the conversion of the airvibration into the heat energy by the viscous effect of the through holein the inside member, and the sufficient sound absorbing performance canbe exerted in the wide frequency band even in an environment in whichthe diameter of the through hole in the second outside member cannotrelatively be decreased. Because the aperture ratio of the secondoutside member becomes not more than 50%, the rigidity of the secondoutside member can be secured to some extent.

In the present invention, it is preferable that the aperture ratio ofthe through hole in the inside member is not more than 3% (claim 7).Therefore, while the inside member has the sufficient sound absorbingperformance, the number of through holes is decreased in the insidemember, so that a production time of the inside member can be shortenedto reduce production cost.

In the present invention, it is preferable that the diameter of thethrough hole in the inside member be not more than 3 mm (claim 8). Thisenables the rapid decrease in acoustic absorption coefficient to besuppressed. Accordingly, the frequency band of the absorbed noise canfurther be widened.

In the present invention, it is preferable that air layers be formedbetween the first outside member and the inside member and between theinside member and the second outside member respectively and a soundabsorbing material be arranged in at least one of the air layers (claim9). This enables the sound absorbing performance to be improved. Glasswool, metal fiber, a foam metal and resin metal thin film or a metalthin film, non-oven cloth, and the like can be utilized as the soundabsorbing material.

In the present invention, it is preferable that air layers be formedbetween the first outside member and the inside member and between theinside member and the second outside member respectively, a soundabsorbing material made of a resin of metal thin film be arranged in atleast one of the air layers, many through holes being made in the soundabsorbing material, the aperture ratio of the through hole of the thinfilm be not more than 3%, and the diameter of the through hole of thethin film be not more than 3 mm (claim 10). Therefore the soundabsorbing performance is further improved.

In the present invention, it is preferable that at least one of thesecond outside member and the inside member be inclined with respect toa horizontal plane (claim 11).

Thus, water can effectively be collected and drained. Additionally, thepainting can effectively be collected and discharged when the poroussound absorbing structure is coated with paint. As a result, thedecrease in sound absorbing material can further be suppressed in themulti-hole plate.

In the present invention, it is preferable that a drain hole be made ina lower portion of at least one of the second outside member and theinside member (claim 12).

Even in the above configuration, the water can effectively be collectedand drained. Additionally, the painting can effectively be collected anddischarged when the porous sound absorbing structure is coated withpaint. As a result, the decrease in sound absorbing material can furtherbe suppressed in the multi-hole plate.

In the present invention, it is preferable that the inside member bemade of a water-repellent material containing at least one of siliconand fluorine or a surface of the inside member be coated with awater-repellent material containing at least one of silicon and fluorine(claim 13).

Therefore, even if rainwater or the like reaches the interior member,the rainwater or the like is effectively removed from the surroundingsof the through hole, so that the rainwater or the like can be preventedfrom adhering to the surroundings of the through hole. As a result, theclogging can further be suppressed in the through hole.

In the present invention, it is preferable that at least one of thefirst outside member, the second outside member, and the inside memberbe formed so as to have an irregular shape (claim 14).

Therefore, at least one of the members is formed so as to have theirregular shape, which allows the rigidity of the member to be enhanced.When the air layer is provided between the first outside member and theinside member or between the second outside member and the inside memberlike claims 9 and 10, the air layer can be formed by overlapping themember, formed so as to have the irregular shape, and other member toeach other.

Preferred embodiments of the present invention will be described belowwith reference to the drawings.

First Embodiment

FIG. 1 is a transverse sectional view showing a porous sound absorbingstructure according to a first embodiment of the present invention. Asshown in FIG. 1, a porous sound absorbing structure 10 according to thefirst embodiment includes an exterior plate (second outside member) 1, aclosing plate (first outside member) 2, and an interior plate (insidemember) 3. The exterior plate 1 is arranged on the sound source side,the closing plate 2 is arranged on the sound insulating side, and theinterior plate 3 is arranged between the exterior plate 1 and theclosing plate 2. An air layer 4 is located between the interior plate 3and the exterior plate 1, and an air layer 5 is located between theinterior plate 3 and closing plate 2. The exterior plate 1, the closingplate 2, and the interior plate 3 are made of a metal such as iron andaluminum, a synthetic resin, a fiber reinforced material, and the like.Desirably the exterior plate 1, the closing plate 2, and the interiorplate 3 are made of the same material because a separation process iseliminated in recycling.

The exterior plate 1 is arranged while opposed to the interior plate 3,and the interior plate 3 is arranged while opposed to the closing plate2. That is, the porous sound absorbing structure 10 is formed while theexterior plate 1, the interior plate 3, and the closing plate 2 areopposed to each other. Many circular through holes 1 a and 3 a are madein the exterior plate 1 and the interior plate 3 respectively, and thethrough hole 1 a of the exterior plate 1 is slightly larger than thethrough hole 3 a of the interior plate 3 in a diameter. Parametersincluding layer thicknesses d1 and d2 of the air layers 4 and 5,aperture ratios β1, and β2 (area ratios) of the exterior plate 1 andinterior plate 3, plate thicknesses t1 and t2, and diameters φ1 and φ2of the through holes 1 a and 3 a are set so as to generate a viscouseffect to air passing through the through holes 1 a and 3 a of theexterior plate 1 and interior plate 3. Thus, because the air vibrationis converted into heat energy by generating a viscous damping effect inthe air passing through the through holes 1 a and 3 a, a dampingproperty is generated in the air vibration. As a result, the high soundabsorbing effect can be exerted in the relatively wide frequency band.Accordingly, the porous sound absorbing structure according to a firstembodiment can deal with the noise having the frequency componentsexcept for the resonance frequency.

Preferably the aperture ratio β1 of the through hole 1 a in the exteriorplate 1 is more than 3% and not more than 50%. This is attributed to anexperimental result (see FIG. 2) that, in the above range, the apertureratio β1 is brought close to or becomes larger than the acousticabsorption coefficient of 0.3 in which the sufficient sound absorbingeffect is obtained. FIG. 2 is a graph showing a relationship between theacoustic absorption coefficient and the aperture ratio β1 of theexterior plate 1 in the porous sound absorbing structure 10 according tothe first embodiment of the present invention. As shown in FIG. 2, whenthe aperture ratio β1 of the through hole 1 a of the exterior plate 1 isabout 5% exceeding 3%, the acoustic absorption coefficient becomes about0.3. When the aperture ratio β1 is 10%, the acoustic absorptioncoefficient becomes 0.35. The acoustic absorption coefficient isgradually decreased when the aperture ratio β1 exceeds 10%, and thesound absorbing effect becomes 0.27 when the aperture ratio β1 is 50%.As can be seen from FIG. 2, from the viewpoint of acoustic absorptioncoefficient, not only the aperture ratio β1 of the exterior plate 1 issimply decreased while exceeding 3%, but also preferably the apertureratio β1 is optimized in the range of 5% to 28% where the acousticabsorption coefficient becomes 0.3 or more in which the sufficient soundabsorbing effect is obtained. More preferably the aperture ratio β1 isoptimized in the range of 7% to 20% where the acoustic absorptioncoefficient becomes 0.33 or more. However, when the aperture ratio β1 is3% or less, because the acoustic absorption coefficient is decreasedsuch that the acoustic absorption coefficient is brought close to 0.2 orbecomes 0.2 or less, the desired sound absorbing effect cannot beexpected. When the aperture ratio β1 exceeds 50%, an aperture area ofthe exterior plate 1 is excessively increased, which leads to thedecrease in rigidity.

FIG. 3 is a graph showing sound absorbing characteristics in the poroussound absorbing structure according to the first embodiment of thepresent invention, and FIG. 3 shows the sound absorbing characteristicswhen the aperture ratio β2 of the interior plate 3 is changed in therange of 0.1% to 5.0% while the aperture ratio β1 of the exterior plate1 is maintained at 2.5%. FIG. 4 is a graph showing the sound absorbingcharacteristics in the porous sound absorbing structure according to thefirst embodiment of the present invention, and FIG. 4 shows the soundabsorbing characteristics when the aperture ratio β2 of the interiorplate 3 is changed in the range of 0.1% to 5.0% while the aperture ratioβ1 of the exterior plate 1 is maintained at 5%. FIG. 5 is a graphshowing the sound absorbing characteristics in the porous soundabsorbing structure according to the first embodiment of the presentinvention, and FIG. 5 shows the sound absorbing characteristics when theaperture ratio β2 of the interior plate 3 is changed in the range of0.1% to 5.0% while the aperture ratio 131 of the exterior plate 1 ismaintained at 10.0%. Table 1 shows conditions of experimental numbers S1to S18 shown in FIGS. 3 to 5.

TABLE 1 Exterior plate Interior plate Plate Diameter of Air layer PlateDiameter of Air layer Experimental thickness through hole Aperturethickness thickness through hole Aperture thickness number t1 (mm) φ1(mm) β1 (%) d1 (mm) t2 (mm) φ2 (mm) β2 (%) d2 (mm) S1 1.5 10.0 2.5 150.3 0.5 0.1 15 S2 * * * * * * 0.2 * S3 * * * * * * 0.5 * S4 * * * * * *1.0 * S5 * * * * * * 2.0 * S6 * * * * * * 5.0 * S7 * * 5.0 * * * 0.1 *S8 * * * * * * 0.2 * S9 * * * * * * 0.5 * S10 * * * * * * 1.0 *S11 * * * * * * 2.0 * S12 * * * * * * 5.0 * S13 * * 10.0  * * * 0.1 *S14 * * * * * * 0.2 * S15 * * * * * * 0.5 * S16 * * * * * * 1.0 *S17 * * * * * * 2.0 * S18 * * * * * * 5.0 * * denotes ditto

On the conditions of the aperture ratios β1 of the exterior plate 1 of2.5%, 5.0%, and 10.0%, when the aperture ratios β2 of the interior plate3 is changed to 0.1%, 0.2%, 0.5%, 1.0%, 2.0%, and 5.0% as shown in Table1, it is found that the frequency band of the noise absorbed by theporous sound absorbing structure 10 is widened as the aperture ratio β1of the exterior plate 1 is increased as shown in FIGS. 3 to 5. That is,when the aperture ratio β1 of the exterior plate 1 is increased from2.5% to 5% and 10.0% in the experimental number S1 like the experimentalnumbers S7 and S13, the frequency band of the absorbed noise is widened,and the noise having the wide frequency band can be absorbed. Similarly,when the aperture ratio β1 of the exterior plate 1 is increased from2.5% to 5% and 10.0% in the experimental numbers S2 to S6 like theexperimental numbers S8 to S12 and S14 to S18, the frequency band of theabsorbed noise is widened, and the noise having the wide frequency bandcan be absorbed. From the experimental results shown in FIGS. 3 to 5, inthe porous sound absorbing structure 10, it is found that the frequencyband of the absorbed noise is widened when the aperture ratio β1 of theexterior plate 1 exceeds 3%. Accordingly, when the aperture ratio β1 ofthe exterior plate 1 is set more than 3% and not more than 50%, theporous sound absorbing structure 10 sufficiently has the sound absorbingperformance while the frequency band of the absorbed noise is widened,and the porous sound absorbing structure 10 secures the rigidity of theexterior plate 1 to some extent. The rigidity of the exterior plate 1 isincreased as the aperture ratio β1 of the exterior plate 1 is decreased.

Preferably the diameter φ1 of the through hole 1 a in the exterior plate1 is more than 3 mm. This is because that the through hole 1 a isclogged in the actual use when the diameter φ1 of the through hole 1 ain the exterior plate 1 is not more than 3 mm. Further, sometimes thediameter φ1 of the through hole 1 a in the exterior plate 1 cannot bemade not more than 3 mm due to constraints such as drainage and paintingwhich are of other functions of the exterior plate 1. Accordingly, whenthe diameter φ1 of the through hole 1 a in the exterior plate 1 exceeds3 mm, the above problems can be avoided.

Preferably the aperture ratio β2 of the through hole 3 a in the interiorplate 3 is not more than 3%. As shown in FIGS. 4 and 5, when theaperture ratio β2 of the interior plate 3 is more than 3%, namely, inthe case of the experimental numbers S12 and S18, the acousticabsorption coefficient becomes not more than 0.3, so that theinsufficient sound absorbing effect is obtained. On the contrary, whenthe aperture ratio β2 of the interior plate 3 is not more than 3%,namely, in the case of the experimental numbers S7 to S11 and S13 toS17, the acoustic absorption coefficient near a particular resonancefrequency becomes not lower than 0.3, so that the noise can sufficientlybe absorbed.

On the other hand, in FIG. 3, the acoustic absorption coefficientexceeds 0.3 in any one of experimental numbers S1 to S6. However,because the aperture ratio β1 of the exterior plate 1 becomes not morethan 3%, the frequency band of the absorbed noise is narrowed (that is,a width near the particular resonance frequency band is narrowed).

Thus, as shown in FIGS. 4 and 5, when the aperture ratio β1 of theexterior plate 1 exceeds 3% while the aperture ratio β2 of the throughhole 3 a in the interior plate 3 becomes not more than 2%, the acousticabsorption coefficient near the particular resonance frequency bandexceeds 0.3 in the experimental numbers S7 to 11 and S13 to S17. Fromthe results shown in FIGS. 4 and 5, in the porous sound absorbingstructure 10, the aperture ratio β2 of the interior plate 3 is set notmore than 3%, which allows the acoustic absorption coefficient not lowerthan 0.3 to be obtained while the frequency band of the absorbed noiseis widened. When the aperture ratio β2 becomes not more than 3%, becausethe number of through holes 3 a can be decreased, the production time ofthe interior plate 3 can be shortened to reduce the production cost ofthe porous sound absorbing structure 10.

Preferably the diameter φ2 of the through hole 3 a in the interior plate3 is not more than 3 mm. This is attributed to the experimental result(see FIG. 6) that, when the diameter φ2 of the through hole 3 a issmaller, the rapid decrease in acoustic absorption coefficient can besuppressed while the frequency band of the absorbed noise is widened.FIG. 6 is a graph showing the sound absorbing characteristics in theporous sound absorbing structure according to the first embodiment ofthe present invention, and FIG. 6 shows the sound absorbingcharacteristics when the aperture ratio β2 of the interior plate 3 ischanged in the range of 0.1% to 5.0% while the aperture ratio β1 of theexterior plate 1 is maintained at 10% and, at the same time, when thediameter φ2 of the through hole 3 a is maintained at 0.2 mm. Table 2shows conditions of experimental numbers S19 to S24 shown in FIG. 6.

TABLE 2 Exterior plate Interior plate Plate Diameter of Plate Diameterof Plate Diameter of Experimental thickness through hole thicknessthrough hole thickness through hole number t1 (mm) φ1 (mm) t1 (mm) φ1(mm) t1 (mm) φ1 (mm) S19 1.5 10.0 10.0 15 0.1 0.2 0.1 15 S20 * * * * * *0.2 * S21 * * * * * * 0.5 * S22 * * * * * * 1.0 * S23 * * * * * * 2.0 *S24 * * * * * * 5.0 * * denotes ditto.

As shown in Table 2, in the conditions of experimental numbers S19 toS24 of FIG. 6, the plate thickness t2 of 0.3 mm of the interior plate 3in the conditions of the experimental number S13 to S18 of FIG. 5 ischanged to 0.1 mm, and the diameter of 0.5 mm of the through hole 3 a ischanged to 0.2 mm. In the experimental numbers S19 to S24, the rapiddecrease in acoustic absorption coefficient is suppressed on the aboveconditions as shown in FIG. 6. That is, from the comparison of FIGS. 5and 6, the rapid decrease in acoustic absorption coefficient can beprevented by decreasing the diameter φ2 of the through hole 3 a.Preferably, when the diameter φ2 of the through hole 3 a is set not morethan 3 mm, the rapid decrease in acoustic absorption coefficient shownin FIG. 5 can be prevented. Therefore, in the porous sound absorbingstructure 10, the frequency band of the absorbed noise can be widenedwhile the rapid decrease in acoustic absorption coefficient issuppressed.

Thus, the porous sound absorbing structure 10 of the first embodiment isformed such that the aperture ratio β1 of the exterior plate 1 exceeds3%. Therefore, the conversion of the air vibration into the heat energyis promoted by the viscous damping effect of the through hole 3 a in theinterior plate 3, and the sound absorbing performance can sufficientlybe exerted in the wide frequency band. The porous sound absorbingstructure 10 of the first embodiment is formed such that the diameter φ1of the through hole 1 a exceeds 3 mm, and the interior plate 3 isarranged inside the exterior plate 1 so as not to be exposed to theoutside. Therefore, when the porous sound absorbing structure 10 isactually used, the clogging is hardly generated in the through holes 1 aand 3 a, and the constraint on the size of the diameter φ1 caused by thedrainage and painting which are of other functions of the exterior plate1 can be eliminated. Accordingly, the sound absorbing performance ishardly degraded while the frequency band of the noise absorbed by theinterior plate 3 of the porous sound absorbing structure 10 is widened.Because the aperture ratio β1 of the through hole 1 a in the exteriorplate 1 exceeds 3% while the diameter φ1 of the through hole 1 a exceeds3 mm, even in an environment in which the diameter φ1 of the throughhole 1 a in the exterior plate 1 cannot relatively be decreased, theporous sound absorbing structure 10 which securely exerts the sufficientsound absorbing performance in the wide frequency band can be obtained.

The aperture ratio β2 of the interior plate 3 becomes not more than 3%,which enables the porous sound absorbing structure 10 to have thesufficient sound absorbing performance. The number of through holes 3 acan be decreased in the interior plate 3, so that the production time ofthe interior plate 3 can be shortened to reduce the production cost. Thediameter φ2 of the through hole 3 a in the interior plate 3 is set notmore than 3 mm, which allows the rapid decrease in acoustic absorptioncoefficient to be suppressed in the porous sound absorbing structure 10.Therefore, the frequency band of the noise absorbed by the porous soundabsorbing structure 10 can further be widened.

Second Embodiment

FIG. 7 is a transverse sectional view showing a porous sound absorbingstructure according to a second embodiment of the present invention. Asshown in FIG. 7, a porous sound absorbing structure 20 of the secondembodiment has the substantially same configuration as the porous soundabsorbing structure 10 of the first embodiment except that a glass wool(sound absorbing material) 21 is arranged in the air layer 5 of theporous sound absorbing structure 10 of the first embodiment. In FIG. 7,the same component as the first embodiment is designated by the samenumeral, and the description will be omitted.

In the second embodiment, although the glass wool 21 is provided in theair layer 5, for example, the sound absorbing material made of the metalfiber, the foam metal and resin metal thin film or the metal thin film,or the non-oven cloth may be provided in the air layer 5. The glass wool21 may be provided not only in the air layer 5 but in the air layer 4,or the glass wool 21 may be provided only in the air layer 4. The glasswool 21 is provided in the air layer 5, and thereby the air vibrationpassing through the through holes 1 a and 3 a of the exterior plate 1and interior plate 3 is further damped by the glass wool 21. That is,the air vibration damped by the viscous damping effect is further dampedby the glass wool 21 when the air vibration passes through the throughhole 3 a in the interior plate 3.

The substantially same effects as the porous sound absorbing structure10 of the first embodiment can also be obtained in the porous soundabsorbing structure 20 of the second embodiment. Because the soundabsorbing effect by the glass wool is added to the porous soundabsorbing structure 20, the sound absorbing performance is improvedcompared with the porous sound absorbing structure 10 of the firstembodiment.

Third Embodiment

FIG. 8 is a transverse sectional view showing a porous sound absorbingstructure according to a third embodiment of the present invention. Asshown in FIG. 8, a porous sound absorbing structure 30 of the thirdembodiment has the substantially same configuration as the porous soundabsorbing structure 10 of the first embodiment except that multiple thinfilms 31 are arranged so as to vertically partition the air layer 5 ofthe porous sound absorbing structure 10 of the first embodiment. In FIG.8, the same component as the above-described embodiments is designatedby the same numeral, and the description will be omitted.

In the multiple thin films 31, two thin films 32 and 33 are laminatedwhile being adjacent to each other. Many through holes 32 a and 33 a aremade in the thin films 32 and 33 respectively, and the through holes 32a and 33 a have the same aperture ratios and diameters as the interiorplate 3. As shown in FIG. 8, the through holes 32 a and 33 a in the twothin films 32 and 33 are made while not overlapped to each other whenviewed from a direction in which the thin films 32 and 33 are laminated.That is, the through holes 32 a and 33 a in the thin films 32 and 33 aremade at the positions where the through holes 32 a and 33 a opposed toeach other are not overlapped to each other. The metal thin film such asaluminum foil and the resin thin film such as vinyl chloride can be usedas the two thin films 32 and 33 of the multiple thin films 31. However,the thin films 32 and 33 of the multiple thin films 31 are not limitedto the particular material.

Thus, when the multiple thin films 31 are provided in the air layer 5,the two thin films 32 and 33 of the multiple thin films 31 are vibratedby the air vibration passing through the through holes 1 a and 3 a inthe exterior plate 1 and interior plate 3, and thereby the two thinfilms 32 and 33 are brought into contact with each other and rubbedtogether to damp the air vibration. That is, the air vibration which isdamped by the viscous damping effect passing through the through hole 3a of the interior plate 3 is further damped by the multiple thin films31. Because the through holes 32 a and 33 a are made in the two thinfilms 32 and 33 while not overlapped to each other, the air vibrationflowing into the air layer 5 passes through the through hole 32 a in thethin film 32. Then, the air vibration passes between the thin films 32and 33, and the air vibration passes through the through hole 33 a inthe thin film 33. That is, because the air vibration propagates alongthe inner surfaces of the two thin films 32 and 33, the sound absorbingeffect is further exerted by combining the damping effect in the passageof the air vibration through the through holes 32 a and 33 a and theviscous damping effect in the propagation of the air vibration throughthe surfaces of the thin films 32 and 33. Furthermore, because themultiple thin films 31 does not depend on the frequency band of theabsorbed noise, the noise of the wide frequency band is effectivelydamped.

The substantially same effects as the porous sound absorbing structure10 of the first embodiment can also be obtained in the porous soundabsorbing structure 30 of the third embodiment. Because the soundabsorbing effect by the multiple thin films 31 is added to the poroussound absorbing structure 30, the sound absorbing performance isimproved compared with the porous sound absorbing structure 10 of thefirst embodiment.

Although the multiple thin films 31 including the two thin films 32 and33 in which the through holes 32 a and 33 a are made are applied in theporous sound absorbing structure 30, the multiple thin films includingthe thin films in which the through holes 32 a and 33 a are not made maybe applied. Therefore, as described above, the thin films are broughtinto contact with each other and rubbed together to damp the airvibration. The multiple thin films may be formed by one thin film, orthe multiple thin films may be formed by at least three thin films.Therefore, the air vibration can further be damped to improve the soundabsorbing effect.

Fourth Embodiment

FIG. 9 is a transverse sectional view showing a porous sound absorbingstructure according to a fourth embodiment of the present invention. Asshown in FIG. 9, a porous sound absorbing structure 40 of the fourthembodiment has the substantially same configuration as the porous soundabsorbing structure 30 of the third embodiment except that the glasswool 21 are arranged in the air layer 4 of the porous sound absorbingstructure 30 of the third embodiment. In FIG. 9, the same component asthe above-described embodiments is designated by the same numeral, andthe description will be omitted.

The glass wool 21 is arranged in the air layer 4 and the multiple thinfilms 31 are arranged in the air layer 5, so that the air vibrationpassing through the through hole 1 a in the exterior plate 1 is dampedby the glass wool 21, the air vibration damped by the glass wool 21 isfurther damped when the air vibration passes through the through hole 3a in the interior plate 3, and the air vibration damped by the glasswool 21 and the through hole 3 a in the interior plate 2 is furtherdamped by the multiple thin films 31.

According to the porous sound absorbing structure 40 of the fourthembodiment, the effects of the porous sound absorbing structures 10, 20,and 30 of the first to third embodiments can be obtained, the soundabsorbing effect is improved in the combination with the effects, andthe frequency band of the absorbed noise is widened in the porous soundabsorbing structure 40.

The porous sound absorbing structures 10, 20, 30, and 40 of the first tofourth embodiments can also be applied to a region where theconventional sound absorbing member is used. For example, the poroussound absorbing structures 10, 20, 30, and 40 of the first to fourthembodiments are used are used as a structure panel of a soundproof fencewhich realizes both the inside sound absorption and the outside soundinsulation for various noise sources such as a motor and a gear. Theporous sound absorbing structures 10, 20, 30, and 40 of the first tofourth embodiments are also used as the sound absorbing plate in a halland a living room.

Fifth Embodiment

FIG. 10 is a transverse sectional view showing a porous sound absorbingstructure according to a fifth embodiment of the present invention. Inthe porous sound absorbing structure 50 of the fifth embodiment, asshown in FIG. 10, while the flat exterior plate 1 and interior plate 3are inclined with respect to the closing plate 2, drain holes are madein lower portions of the inclinations. Other portions of the poroussound absorbing structure 50 of the fifth embodiment are substantiallysimilar to the porous sound absorbing structure 10 of the firstembodiment. In FIG. 10, the same component as the above-describedembodiments is designated by the same numeral, and the description willbe omitted.

Thus, according to the porous sound absorbing structure of the fifthembodiment, the water can effectively be collected and drained.Additionally, the painting can effectively be collected and dischargedwhen the porous sound absorbing structure is coated with paint. As aresult, the decrease in sound absorbing material can further besuppressed in the multi-hole plate.

It is not always necessary that the exterior plate 1 and the interiorplate 3 be evenly inclined, and the number of drain holes is not limitedto one. For example, in one of modifications of the fifth embodiment, asshown in FIG. 17, inclination orientations of the interior plate 3 arealternately changed, and the plural drain holes are provided in thelower portion of the inclination.

The preferred embodiments of the present invention are described above.However, the present invention is not limited to the above embodiments,but various design changes could be made without departing from thescope of claims. For example, in the exterior plate 1 in the poroussound absorbing structures 10, 20, 30, 40, and 50 of the first to fifthembodiments, the aperture ratio β1 exceeds 3% and the diameter of thethrough hole 1 a exceeds 3 mm. However, one of the two conditions may besatisfied in the exterior plate 1 of the porous sound absorbingstructures 10, 20, 30, 40, and 50. The aperture ratio β2 may exceed 3%in the interior plate 3. The diameter φ2 may exceed 3 mm in the throughhole 3 a of the interior plate 3. The plural interior plates 3 may bearranged between the exterior plate 1 and the closing plate 2 whilebeing adjacent to one another, which allows the viscous damping effectto be improved in the through hole of the interior plate to enhance thesound absorbing performance of the porous sound absorbing structure.

The exterior plate 1, the closing plate 2, and the interior plate 3 inthe above embodiments are formed in the rectangular flat shape. However,for example, the exterior plate 1, the closing plate 2, and the interiorplate 3 may be formed in a circular shape, an elliptic plane, or atriangular plane. Preferably the irregularities are formed in theexterior plate 1, the closing plate 2, and the interior plate 3, becausethe irregularity has the effect of enhancing the rigidity. When theirregularities are formed in the exterior plate 1, the closing plate 2,and the interior plate 3, the air layers can be provided among theexterior plate 1, the closing plate 2, and the interior plate 3 only byoverlapping the exterior plate 1, the closing plate 2, and the interiorplate 3 to one another. The exterior plate 1 and the interior plate 3may be formed not in the flat plates but in the thin films. For example,the metal thin film such as aluminum foil and the resin thin film suchas vinyl chloride can be applied to the exterior plate 1 and theinterior plate 3. The closing plate 2 may be formed in an annular shapein section.

For example, FIGS. 11 to 17 are explanatory views showing modificationsof the exterior plate 1 and the interior plate 3 in the embodiments.FIG. 11 shows a state in which irregularity is formed in the exteriorplate 1, FIG. 12 shows a state in which the irregularity is formed inthe interior plate 3, FIG. 13 shows a state in which the irregularity isformed in the exterior plate 1 and the interior plate 3, FIG. 14 shows astate in which a corrugated irregularity is formed in the interior plate3, FIG. 15 shows a state in which an outside rib 1 b and an inside rib 3b are provided in the exterior plate 1 and the interior plate 3 and theirregularities are formed in the exterior plate 1 and the interior plate3 respectively, FIG. 16 shows a state in which the corrugatedirregularity is formed in the interior plate 3 and the emboss isprovided in the exterior plate 1 while the irregularity is formed in theexterior plate 1, and FIG. 17 shows a state in which the irregularitiesare formed in the exterior plate 1 and the interior plate 1 when theinterior plate 3 is inclined.

The inside member may be made of a water-repellent material containingat least one of silicon and fluorine, or a surface of the inside memberis coated with a water-repellent material containing at least one ofsilicon and fluorine.

Therefore, even if the rainwater or the like reaches the interiormember, the rainwater or the like is effectively removed from thesurroundings of the through hole, so that the rainwater or the like canbe prevented from adhering to the surroundings of the through hole. As aresult, the clogging can further be suppressed in the through hole.

In the conventional porous soundproof structure, because the interiorplate is exposed onto the sound source side, there is a possibility thatthe clogging is generated in the through hole by the rainwater, dust,and the like which pass through the hole of the second exterior memberto adhere to the interior member. Accordingly, even if the rainwater orthe like reaches the interior member, the adhesion of the rainwater, thedust, and the like to the interior member can further be suppressed whenthe surface of the interior member has the water repellency. In thiscase, it is particularly necessary that the interior member surfaceincluding the inner surface of the hole have the water repellency. Whenthe interior member surface including the inner surface of the hole hasthe water repellency, even if the rainwater or the like reaches theinterior member, the rainwater can be prevented from invading into thehole. On the other hand, when the interior member surface including theinner surface of the hole does not have the water repellency, therainwater or the like is sucked into the hole by capillary force, whichresults in the clogging.

As used herein, “having the water repellency” shall mean that a contactangle becomes more than 90° when a water droplet is caused to fall onthe surface of the member. The contact angle more than 90° means thatthe state in which the surface of member is in contact with the water ismore unstable than the state in which the surface of the member isexposed to atmospheric air. On the contrary, the contact angle lowerthan 90° means that the state in which the surface of member is incontact with the water is more stable than the state in which thesurface of the member is exposed to the atmospheric air. In order tosuppress the invasion of the rainwater or the like into the hole of theinterior member, the contact angle is not lower than 90°, preferably notlower than 100°, more preferably not lower than 110°.

In order that the interior member surface including the inner surface ofthe hole has the water repellency, the interior member may be made ofthe material having the water repellency, or the surface of the metalmember such as aluminum may be applied or coated with the materialhaving the water repellency. A material having a poly-alkyl siloxanestructure represented by silicone resin, a silane compound having anorganic functional group bonded to a silicon atom represented by silanecoupling agent through a carbon atom, a material havingfluorohydrocarbon structure represented by fluororesin, a fluoro-silanecoupling agent in which a carbon fluoride chain is introduced to thesilane coupling agent, and mixtures thereof can be applied to thematerial having the practical water repellency include. These compoundscan solely be used, or the compounds can be used while at least twocompounds are mixed together. These compounds may be applied as thewater-repellent material by mixing the compound in another vehicle whichdoes not contain the silicon and fluorine as long as the contact anglecan be maintained not lower than 90° between the water and the interiormember.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the apertureratio of the second outside member exceeds 3%, which promotes theconversion of the air vibration into the heat energy by the viscouseffect of the through hole in the inside member, and the sufficientsound absorbing performance can be exerted in the wide frequency band.Because the aperture ratio of the second outside member becomes not morethan 50%, the rigidity of the second outside member can be secured tosome extent.

1. A porous sound absorbing structure which includes: a first outsidemember; a second outside member which is arranged while opposed to saidfirst outside member, many through holes being made along a flat planein said second outside member; and at least one inside member which isarranged while opposed to said first outside member and said secondoutside member such that said inside member is located between saidfirst outside member and said second outside member, many through holesbeing made along the flat plane in said second outside member, theporous sound absorbing structure characterized in that a plate thicknessof said inside member and a diameter and an aperture ratio of thethrough hole are set so as to generate an viscous effect in air passingthrough said through hole, the aperture ratio of the through hole insaid second outside member is more than 3% and not more than 50%, andthe diameter of the through hole in said second outside member exceeds 3mm, wherein a drain hole is made in a lower portion of at least one ofsaid second outside member and said inside member.
 2. The porous soundabsorbing structure as in claim 1, characterized in that the apertureratio of the through hole in said inside member is not more than 3%. 3.The porous sound absorbing structure as in claim 1, characterized inthat the diameter of the through hole in said inside member is not morethan 3 mm.
 4. The porous sound absorbing structure as in claim 1,characterized in that air layers are formed between said first outsidemember and said inside member and between said inside member and saidsecond outside member respectively, and a sound absorbing material isarranged in at least one of said air layers.
 5. The porous soundabsorbing structure as in claim 1, characterized in that air layers areformed between said first outside member and said inside member andbetween said inside member and said second outside member respectively,a sound absorbing material made of a resin of metal thin film isarranged in at least one of said air layers, many through holes beingmade in said sound absorbing material, the aperture ratio of the throughhole of said thin film is not more than 3%, and the diameter of thethrough hole of said thin film is not more than 3 mm.
 6. The poroussound absorbing structure as in claim 1, characterized in that at leastone of said second outside member and said inside member is inclinedwith respect to a horizontal plane.
 7. The porous sound absorbingstructure as in claim 1, characterized in that a drain hole is made in alower portion of at least one of said second outside member and saidinside member.
 8. The porous sound absorbing structure as in claim 1,characterized in that said inside member is made of a water-repellentmaterial containing at least one of silicon and fluorine, or a surfaceof said inside member is coated with a water-repellent materialcontaining at least one of silicon and fluorine.
 9. The porous soundabsorbing structure as in claim 1, characterized in that at least one ofsaid first outside member, said second outside member, and said insidemember is formed so as to have an irregular shape.